It is an effective way to solve the current energy crisis and environmental deterioration by micro-grid acceptance and using of distributed power (photovoltaic power, wind power, fuel cells and the like), where an inverter in the micro-grid is used as an interface of the distributed power and the micro-grid to convert distributed energy into high-quality electric energy, therefore researches on inverters which are suitable for the operation of the micro-grid is of great significance. The current researches mainly focus on control methods when the inverters work in a grid-connected mode or an off-grid (island) mode separately, but for inverters capable of working in the grid-connected and off-grid dual-mode, researches on corresponding smooth switch control thereof are not perfect.
In a highly flexible micro-grid, the inverter should have the ability to simultaneously provide electric energy to a local load and the power grid, and the inverter can be disconnected from the power grid and directly supply power to the local load under exceptional conditions. During grid-connected switch, instant overvoltage or over current will be generated because the micro-power source and the power grid cannot be strictly synchronized and differences exist in control schemes; during off-grid switch, the three phases of a thyristor (SCR) or a solid-state relay (SSR) cannot be cut off at the same time, resulting in imbalance in voltage or current, so that the fluctuation of active or reactive power is increased to cause instability of a DC-side voltage. These factors influence the normal work of the load and break the reliability and stability of power supply. The existing dual-mode inverters generally adopt P/Q control at the grid-connected mode and adopt V/f control at the off-grid mode. Although transition control is adopted to inhibit the impact of the output current of the inverter in a switching process, only the fluctuation of the output voltage, current and power of the inverter is concerned, but the reduction of the impact of the grid current is not taken into account. The droop control at the grid-connected mode and the off-grid mode can achieve steady-state control of the system and guarantee the equipartition of load and power. But for dual-mode switching control, only the existing smooth switching mode can be depended on, and smooth switching cannot be carried out according to the characteristics of droop control, thereby lacking pertinence.